Factors influencing BLR

6.6.1 Effect of initial load and temperature

The initial load and the operational temperature during bolt load retention test play very important role in determining the degree of clamp load relaxation at bolted joints. The higher the load, the more the stress acting on the specimen and this invariably leads to more deformation on the sample. The result is loss of fastener clamp load at such a joint. Figure 51 shows the influence of initial load (Pi) and temperature on the bolt load retention response of AS41. The symbol Co represents the ratio of PR to that of PK expressed in percentage. This indicates the degree of load retained at test temperature. It could be seen from Figure 51 that the higher the temperature, the more the relaxation experienced by the Mg-alloy. From Figure 51, it is observed that at low stress level and moderate temperature, up to 80 % of the load was retained at test temperatures. Loss of approximately 30 % was experienced for loads of 10 kN and 17 kN which corresponds to 40 MPa and 70 MPa at 150 °C. The load loss increased to 36 % and 42 % at 175 °C for 10 kN and 17 kN load respectively.

100 120 140 160 180

0 20 40 60 80 100

5 kN 10 kN 17 kN Co=(PR/PK)

Temperature (°C)

Figure 51: Effect of initial load and temperature on BLR of AS41 Mg-alloy

The same is also true when CT as shown in Figure 52 is plotted against temperature for respective initial loading conditions. CT is the sum of the total bolt load retained on the bolted couple throughout the whole process. This is computed by taking the ratio of the final load PF to that of PI. This gives an overview of the load behaviour between PI and PF after the sample has suffered creep between PK and PR. Figure 52 shows a plot of CT against temperature.

The temperatures here represent the test temperature before cooling the bolted couple back to PF at room temperature.

100 125 150 175 0

20 40 60 80 100

5 kN 10 kN 17 kN

CT=(PF/PI)

Temperature (°C)

Figure 52: Effect of initial and final load on BLR of AS41 Mg-alloy

From Figure 52, one observes more than 50 % load loss when specimens were cooled down to room temperature. This is for 150 °C and 175 °C in all initial load conditions investigated. At 125 °C, more than 50 % of the initial load was retained for stresses of 40 MPa and 70 MPa on the AS41 Mg-alloy. Generally speaking, the effect of deformation suffered by the AS41 sample during the isothermal phase of PK and PR were reflected on the overall BLR results of AS41. In this investigation, the higher the temperature and load, the more the bolt load loss for permanent mould AS41 alloy.

6.6.2 Influence of effective length

In order to investigate the influence of effective length on BLR response of AS41 Mg-alloy, the thickness of the sample was doubled. This means that the effective length of the steel bolt was increased by 15 mm. This increase in length brought the effective length of the bolt to 52 mm. The effective length of the bolt is defined here as the sum of the engagement length plus 0.5 of the head bolt and nut. The term H in Figure 53 and Figure 54 stands for the height of the AS41 sample used in this investigation. From the experimental results in Figure 53, it is observed that the couple with longer effective length show higher bolt load value as compared to that of shorter effective length. An increase of 31

% in bolt load is witnessed at Pk with respect to the initial preload of 17 kN for

6 Results 81

the longer effective length couple. In the case of shorter effective length couple, an increase of 20.4 % was recorded at Pk for the same initial preload.

0 20 40 60 80 100

Figure 53: Plot of effective lengths at 150 °C for loads of 5 kN and 17 kN

0

Figure 54: Plot of retained load at 150 °C for longer and shorter engagement length

From Figure 53, an average difference of 2 kN exists between longer and shorter engagement length with respect to the measured bolt load at test temperature as can be seen from the experiment. The percentage bolt load retained for both longer and shorter lengths after 72 hours at 150 °C were close to each other at different preload. At 17 kN preload, a Pr/Pk ratio of 74 % is measured for the longer length as compared to 71 % for the shorter length. For the preload of 10 kN, 75 % load was retained for the longer length while 73 % was recorded for the shorter length. At 5 kN preload, 80 % of the load was

retained as against 78 % for the longer and shorter effective length respectively.

Figure 54 shows a comparative plot of the retained load at 150 °C for AS41 alloy. It could be said that the longer effective length retained slightly higher bolt load than that of the shorter.

6.6.3 Effect of bolt stiffness

The stiffness of a joint is determined mainly by the effective elastic moduli of the joint members. In this investigation, high Young’s modulus of either bolt or nut results in higher fastener clamp load retention at test temperature. The result of a 30 mm height and 20 mm diameter AS41 tested with steel bolt/ steel nut couple were compared to that of aluminium bolt/ aluminium nut joint. The BLR profile measured for this test is shown in Figure 55 and Figure 56.

0 20 40 60 80 100

Figure 55: Steel bolt-Aluminium bolt joint tested at 10 kN 125 °C

It could be seen from the plot that the stiffer steel joint retained more load than the less stiff aluminium joint at test temperature. A ratio of Pr/Pk shows clamp load retention of 69 % and 82 % for steel joint at 17 kN and 10 kN initial load respectively. That of aluminium shows 58 % and 64 % for the same condition of initial loads of 17 kN and 10 kN respectively. That is 11 % difference in retained bolt load for the 17 kN preload condition and 18 % difference for the 10 kN preload. The load retained for the 10 kN preload is however larger than that of

6 Results 83

17 kN initial load in both the stiffer steel bolted joint and in aluminium bolted joint.

0 20 40 60 80 100

0 20 40 60 80 100

P_(T)Al

(MPa)

Time (H)

Al bolt/nut Steel bolt/nut

P_(T)St

T=125 °C P_I=17 kN

0 4 8 12 16 20 24

Load (kN)

Figure 56: Steel bolt-Aluminium bolt joint tested at 17 kN 125 °C

The total load change ∆P(T) which is the difference in bolt load from the highest load reached at test temperature (Pk) and the final load (PF) at room temperature increased for steel bolted couple. At 17 kN preload and temperature of 125 °C, ∆P(T)Al was 65.9 % while ∆P(T)Steel was 75.6 %. For the 10 kN preload at 125 °C, ∆P(T)Al and ∆P(T)Steel were 81.7 % and 80.4 % respectively.